Extremely slow earthquakes occurred at the Nankai Trough, south of Japan, in March 2009, Japanese researchers have reported.
Crustal ruptures in the earthquakes took 30 to 100 seconds, compared with some 1 to 2 seconds in ordinary quakes with magnitude of around 4, said the researchers, including members of the Japan Agency for Marine-Earth Science and Technology, or JAMSTEC.
The findings were published in the latest online edition of the British journal Nature Geoscience.
The low-frequency earthquakes took place in unexpectedly shallow areas of the plate boundary, suggesting such areas may be a source of so-called tsunami earthquakes that cause disproportionately large tsunami for their seismic energy, the researchers said.
By setting up broadband seismometers off the coast of Tanabe, in Wakayama Prefecture, they observed the earthquakes for 10 days from March 22, 2009.
“Our research could lead to understanding the mechanism of tsunami quakes, which could cause severe damage, and it is necessary to continue the research for the long run,” said Hiroko Sugioka, a JAMSTEC researcher.